Control of nuclear reactors

ABSTRACT

In a coupling system for a nuclear reactor control rod and control rod drive, said coupling system being selectively uncouplable by an uncoupling rod, a system of passages for exercising a countervailing force on the uncoupling rod to prevent inadvertent uncoupling during scram.

BACKGROUND AND SUMMARY

Control rod drives are employed in nuclear reactors to position controlrods with respect to the core of the reactor. Depending upon whethersufficient numbers of control rods are inserted into the core atappropriate locations and whether these rods are completely or partiallyinserted, the reactor is either completely shut down or its operation ismodified to continue at a different level or according to a differentpower distribution within the core. Fast or emergency insertion of thecontrol rods is referred to by those skilled in the art of nuclearreactors as "scram." Normal insertion or withdrawal of the control rodsin the reactor, on the other hand, is called "shimming."

Control rod drives for electromechanically establishing the level ofcontrol rods in a reactor for normal operation, optimal power shaping,and economical fuel management have been known for years. One such driveis the subject of a patent application in West Germany by KraftwerkUnion AG. Offenlegungschrift Pat. No. 2442722 contains the applicationwhich was laid open to the public on Mar. 25, 1976.

This drive is not sectionable into parts for removal, however, making itnecessary for the entire drive to be removed in one piece from thereactor pressure vessel for maintenance, repair, or disassembly.Furthermore, this drive does not employ a differential pressuretechnique as disclosed herein to prevent inadvertent uncoupling of thecontrol rod drive and the control rod itself during reactor operation orscram. These features, among others, serve to distinguish the instantinvention from the apparent prior art.

The invention herein is applicable to boiling water reactors, in whichthe core is covered with water for cooling, and which are designed toinsert control rods from below the pressure vessel containing the coreof the reactor. The claims cover concepts and features, however, whichcan be employed in other kinds of reactors and in linear-motionproducing devices not within the field of reactor technology.

For more details with regard to the kinds of nuclear reactors, see pages5-1 to 5-129 in Energy Technology Handbook (1977) by McGraw-Hill,[Douglas M. Considine, P.E., editor-in-chief]. Additionally, NuclearReactor Engineering by Samuel Glasstone and Alexander Sesonske,published in 1981 by Van Nostrand Reinhold Company, and Introduction toNuclear Engineering by John R. Lamarsh, published in 1977 byAddison-Wesley Publishing Company, are useful references.

The reactor pressure vessel in a boiling water reactor is supported by amassive foundational structure or pedestal, perhaps constructed ofconcrete. The structure defines an opening or "hatch" which permits theessentially horizontal transportation of the control rod drive betweenthe underside of the pressure vessel and other compartments of thereactor facility.

The underside of the pressure vessel defines a pit of predetermineddimensions and is bounded by the walls of the pedestal supporting thepressure vessel. Numerous control rod drives (their precise numberdepends upon the size of the reactor core and the number of control rodsneeded to control criticality of the reactor) extend below the pressurevessel into the pit.

Each control rod drive is suitably coupled to a control rod forcontributing to control of the reactor. The core is suitably high in thepressure vessel to permit the control rods to remain within the pressurevessel when fully withdrawn from the core. This requirement makes itnecessary for a portion of the control rod drive to traverse thedistance between the bottom of the pressure vessel to the vicinity ofthe core, in order to fully insert a control rod within the core. Whenthis portion of the control rod drive is fully withdrawn from thepressure vessel, it is contained within a pressure tube of the controlrod, which is typically about 16 feet long.

In fully hydraulically operated control rod drives, the motive power forscram or partial insertion of control rods into the core is appliedthrough a relatively short package of equipment mounted under thepressure vessel. Electromechanical control rod drives are much longer,because they utilize an electric motor at the lower end of the controlrod drive to turn a shaft and spindle to insert the control rod into thereactor core for shimming and which utilize hydraulic drive for scram.In addition, seals and various coupling devices increase the overalllength of the control rod drive.

Installation and removal of such a control rod drive is accomplished bytranslating the aspect of the control rod drive between vertical andhorizontal postures. Passage through the hatch in the pedestal ishorizontal; however, the control rod drive is vertical when installed.Within the limited space available in the pedestal under the pressurevessel, the control rod drive is turned from one aspect to another.Clearly, with a longer control rod drive this is made more difficult andpossibly impossible.

Even when installation and removal of the control rod drive isaccomplished within the bounds and dimensions of the pedestal,preventing the uncoupling of the control rod drive from the control roditself during reactor operation or scram remains a significantchallenge.

The electromechanical control rod drive of interest herein "scrams" byapplying high pressure directly under the head of a hollow drive pistonwhich carries the control rod. In contrast to some kinds of fullyhydraulic control rod drives such as are disclosed in U.S. Pat. Nos.3,020,887 and 3,020,888 (hereby expressly referred to and incorporatedherein), in which the insertion pressure is applied primarily at thefoot of the piston (or index tube), the pressure here is applieddirectly under the head of the piston in the structure discussed withreference to an embodiment of the invention herein.

The uncoupling rod for detaching the control rod drive from the roditself is designed to traverse the head of the drive piston. In the caseof fully hydraulic drives, leakage through the aperture receiving theuncoupling rod is of no great detriment.

However, in the case of the electromechanical control rod drive,excessive leakage through the piston head would prevent theestablishment of a scram-suitable pressure. To control leakage,circumferential ridges and depressions are accordingly formed along thesides of the uncoupling rod. The rod itself is enclosed within the headof the aforementioned drive piston by a second smaller piston which issimilarly formed with circumferential ridges and depressions, whichinter alia help prevent adverse effects due to impurities, particles,and the like which may be found in the water.

To disassemble the control rod, the uncoupling rod is axially translatedin an upward direction. Within the tight tolerance required to preventleakage, water displaced by the movement of the second piston enters orleaves the chamber region by passages provided in the head of the drivepiston.

Another challenge is encountered regarding high pressure scram. Thepressure under the head of the first piston tends to drive theuncoupling rod upward, causing the control rod and the control rod driveto uncouple. Furthermore, at the end of a stroke, portions of thecontrol rod drive including the piston decelerate so rapidly that theinertia of the uncoupling rod causes the rod to destabilize andpotentially separate the control rod from the control rod drive.

OBJECTS OF THE INVENTION

It is an object of the instant disclosure to describe a control roddrive that is conveniently installable, maneuverable under and removablefrom the underside of a nuclear reactor pressure vessel.

It is an object to reduce the size of pressure vessel foundations innuclear reactors by providing a control rod drive in plural sectionswhich are conveniently installable, maneuverable under, and removablefrom the underside of a nuclear pressure vessel.

It is an object to provide a method for conveniently installing andremoving an electromechanical control rod drive vis-a-vis the undersideof a nuclear reactor pressure vessel.

It is an object to prevent the accidental uncoupling of control roddrives from control rods during reactor operation or scram.

A further object of the instant disclosure is to describe a decouplablecontrol apparatus for a nuclear reactor, which is constructed inremovable sections and defines sealing surfaces for preventing thespillage of water.

DESCRIPTION OF THE DRAWING

FIG. 1 is a vertical cross section of a nuclear reactor pressure vesselsupported by a foundational structure;

FIG. 2 is a vertical cross section of a control rod drive disposed onthe underside of the pressure vessel;

FIG. 3 is a horizontal cross section of the control rod drive depictinga ball-screw-and-nut assembly and an anti-rotation roller for preventingthe nut from rotating within the control rod drive; and

FIGS. 4A and 4B illustrate in respective horizontal and verticalcross-sections a spline connection between the shaft of the control roddrive motor and the ball screw.

PREFERRED EMBODIMENT OF THE INVENTION

FIG. 1 shows a pressure vessel 11 of a nuclear reactor 12 mounted on apedestal or foundation 14. The pedestal has an opening or hatch 15 topermit the passage of a control rod drive 16 through pedestal duringinstallation or removal thereof.

The pressure vessel 11 supports a nuclear core 17 containing a nuclearmass capable of attaining criticality. Above-referenced U.S. Pat. Nos.3,020,887 and 3,020,888 provide examples of such a pressure vessel.Control rod drives 16 are effective for inserting control rods 16' intothe core 17 for shutting down the reactor 12 or for controlling thesteam production, fuel burnup, or pressure level thereof.

FIG. 1 shows an outer tube 21 of the control rod drive 16 mounted on thepressure vessel 11. A housing structure 22 and a motor 24 are mounted atthe lower end thereof, and the motor 24 provides fine motion controlover the axial position of the control rod 16'.

The pedestal 14 in FIG. 1 defines a pit 26 of limited dimensions. Forremoval and insertion of a control rod drive 16, a support structure 27is provided to enable a cart 28 to enter the pit 26. The supportstructure 27 may for example be disc-like with a wide gap (not shown)along its diameter. The structure 27 includes a pair of rails 29,straddling the wide gap and supporting wheels 31 of the cart 28. Thecart 28 itself has a longitudinal gap (not shown) between its sides,which permits a control rod drive 16 to be inserted or removedtherebetween as shown at dotted lines 16" for removal or installation.

When the control rod drive 16 is inserted or removed, it must clear thebottom of the pit 26 and the pedestal sides 14". After the control roddrive 16 has been installed into the outer tube 21, the housingstructure 22 and motor 24 can be installed.

The cart 28 includes a pivotally supported body 32 which can bevertically disposed in order to align with the position of a control roddrive 16 mounted in the pressure vessel 11. So disposed, the control roddrive 16 indicated by dotted lines 16" extends below the rails 29 andthrough the wide gap therein mentioned above.

The structure 27 turns on wheels 35 supported on a circular track 36mounted on suitable extensions 37 from the walls of the pedestal 14.This permits the structure 27 to suitably position a control rod drive16 under any desired one of the control rod positions on the undersideof the pressure vessel 11.

In FIG. 2, the outer tube 21, the housing structure or housing 22, and amotor support structure 38 are shown held together by threaded studs 41including a widened portion 42 and nuts 43. These are convenientlyactuable from below. The outer tube 21, the housing 22, and the motorsupport structure 38 include respective flanges 21', 22', and 38' forreceiving the studs 41. A passage 51 is defined in the housing structure22 for applying a scram or high pressure on the underside of a drivepiston 52 supporting the control rod 16'. The drive piston 52accordingly separates regions of relatively high and low pressure. Thehigh and low pressure is suitably distributed in accordance with sealsshown in FIG. 2. The housing 22 holds the drive piston 52 in positionunder the pressure vessel 11. The piston 52 is for example supported bya ball nut 53 depicted in greater detail in FIG. 3 which shows therecirculating ball bearings 53' of the ball nut 53. The ball nut 53 isaxially driven by a threaded screw or spindle 54. The ball nut 53 isprevented from rotating with the spindle 54 by an anti-rotationalbearing 55 connected to the ball nut 53 by a suitable extension 56. Thebearing 55 rides in a vertical slot 57 formed in a rack 58 mounted on aguide tube 59 of the control rod drive 16 whereby rotation of nut 53 isprevented. The drive piston 52 reposes on the ball nut 53 but it is notattached thereto. Thus when the spindle 54 is rotated in the directionto move the ball nut 53 upward, the piston 52 is driven upward. When thespindle 54 is rotated in the direction to move the ball nut 53 downward,the piston 52 follows the ball nut 43 downward by force of gravity. Theguide tube 59 serves as a thermal shield to protect the control roddrive from temperature fluctuations and to guide the axial motion of thedrive piston 52. Additionally, the guide tube 59 permits convenientinsertion and removal as a package of otherwise loose internal parts ofthe control rod drive 16.

FIGS. 2 and 3 further show suitably mounted seals 66 and rollers 67between the guide tube 59 and the drive piston 52. The seals 66 permitmovement of the drive piston 52, yet maintain a separation of high andlow pressure fluids on opposite sides of the drive piston 52. Therelatively low pressure on the upperside of piston 52 is essentiallyreactor pressure. Conversely, the underside of piston 52 is atrelatively high pressure when the piston 52 is driven hydraulicallyduring reactor scram in which event the high pressure fluid admitted tothe underside of drive piston 52 lifts the piston 52 off the ball nut 53and drives the piston 52 upward for rapid insertion of the control rod16' into the fuel core 17.

The drive piston 52 of the control rod drive 16 is coupled to thecontrol rod 16' by a connecting member such as for example a spud 71shown at the top of FIG. 2, which in the case of this embodimentincludes six fingers 72 having knuckles 72'. These are received in asuitable receiving socket of the recess 73 defined in the underside ofthe control rod 16'. More particularly, the fingers 72 of the spud 71bear against the inner walls of the recess 73, thus coupling the spud 71and control rod 16' together in the socket. To prevent the fingers 72 ofthe spud 71 from moving inwardly and releasing the drive piston 52, adownwardly biased plug 75 is inserted between the fingers 72, latchingor locking them in place. The downward bias may for example be providedby, a spring 76. The lower end of the spud 71 is mounted on an uppersubstantially-solid portion or head 52' of the drive piston 52, as forexample by threaded insertion thereinto.

The plug 75 can be axially translated or dislodged to uncouple the drivepiston 52 of the control rod drive 16 from the control rod 16' itself.This is accomplished by upwardly pushing the plug 75 by an uncouplingmember or rod 77 such as that shown in FIG. 2. This rod 77 preferablyextends through an axial hole machined in the drive piston 52.

The uncoupling rod 77 is subject to a destabilizing pressuredifferential established by high and low pressure regions during reactorscram, insofar as a horizontal component of its surface areas issuddenly exposed to the respective pressure regions. As seen in FIG. 2,the top and bottom ends of the uncoupling rod 77 are subject to thereactor scram pressure differential applied across the drive piston 52.The respective top and bottom surfaces of the uncoupling rod 77 engageplug 75 and the spindle 54 for uncoupling the drive piston 52 from thecontrol rod 16' as described hereinafter.

The uncoupling rod 77 includes between its ends an enlarged or widenedportion such as for example a chamber piston 78, which is wider than theremainder of the uncoupling rod 77. Whatever shape is chosen by oneskilled in the art of design and making such a structure, the uncouplingrod 77 should at least be elongated, have ends extending through thedrive piston 52 into both high and low pressure regions, and include acentrally enlarged portion defining upwardly and downwardly orhorizontally disposed surface areas upon which pressurized fluids canact.

FIG. 2 shows the uncoupling rod 77 disposed along the vertical axis ofthe control rod 16' and movable within limits along the axis. Inparticular, the chamber piston 78 is constrained to reciprocate withinthe limits of an elongated cylinderical chamber 79 formed in the head52' of the drive piston 52, at least according to one embodiment. Thebottom of the chamber 79 is closed by a plug 80 formed with an axialhole for insertion therethrough of the lower portion of the uncouplingrod 77. The lower portion of the rod 77 thus extends downward forengagement with the upper end of the spindle 54. As noted above, theupper portion of the rod 77 extends upward through an axial hole in thehead 52' of the drive piston 52 for engagement with the underside of theplug 75 between the fingers 72 of spud 71. Since this plug 75 preventsuncoupling of the control rod 16' and the control rod drive 16 as longas the plug 75 is disposed between the fingers 72, it can be seen thatthe upward movement of the spindle 54 acting through the uncoupling rod77 forces the plug 75 upward out of engagement with the fingers 72 ofthe spud 71 whereby the fingers 72 can move inward to release thecontrol rod drive 16 from the control rod 16'.

Initially, during assembly of the drive piston 52 and the control roddrive 16 generally, the uncoupling rod 77 is installed at the head 52'of the piston 52 by removing the plug 80 and then inserting theuncoupling rod 77 through the upper hole in the head of the piston 52until the piston 78 is situated within its chamber 79. Once inserted,the plug 80 is passed like a washer over the lower end of the rod 77 andeffectively closes the lower portion of the chamber 79, encapsulatingthe piston 78 of the rod 77 and restraining it to reciprocable motionwithin the chamber 79.

During scram, the high pressure in region 82' under the head 52' ofpiston 52 which acts upon the underside of uncoupling rod 77, takentogether with the inertia of the uncoupling rod 77 as the control rod16' decelerates rapidly at the end of its insertion stroke, might causesufficient forces to act upon and move plug 75 upward and out ofengagement with the fingers 72, whereby the control rod is uncoupled.

To prevent such uncoupling, a pair of passages, 81 and 82, are provided.Passage 81 is connected to the lower portion or underside of the chamber79 and communicates water from the region 81' to the underside of thechamber piston 78. Passage 82, on the other hand, is connected to theupper portion or upperside of the chamber 79 and communicates water fromthe region 82' to the upperside of piston 78. Accordingly during scram,high pressure water is provided from the underside of the drive piston52 to the upperside of the piston 78 to provide a counterforce to theinertia of the uncoupling rod 77 and to prevent removal of plug 75. Ineach case, passages 81 and 82 bypass the chamber piston 78 tocommunicate between opposing pressure spaces and establish acountervailing force.

The net downward forces on piston 78 and rod 77 must exceed the netupward forces on the exposed generally horizontal surfaces of the rod 77and piston 78. The net downward forces respectively includecontributions from both the high and low pressure areas on oppositesides of piston 52.

The amount by which the net downward forces upon rod 77 and piston 78 inthis embodiment must exceed the net upward force depends on thedeceleration of the drive piston 52 at the top of its stroke and uponthe pressure differential at that stage of scram.

Passages 81 and 82 also allow water to be transferred to and fromchamber 79 as piston 78 reciprocates in chamber 78.

The sides of the uncoupling rod 77 and the piston 78 are defined withcircumferential grooves and ridges in their surfaces. These grooves andridges are preferably annular or circumferential. This minimizes theleakage of water through head 52' and circumventing uncoupling rod 77and piston 78. The grooves and ridges additionally protect the slidingsurfaces of rod 77 and cylinder 78 from galling.

The motor 24 shown in FIG. 2 controls the rotation of the spindle 54 byturning a shaft 83 which connects to the spindle 54 through a splineconnection 84 shown in greater detail in FIGS. 4A and 4B. The splineconnection 84 includes an upper portion 85, formed on the lower end ofspindle 54, which is cooperatively recieved by a lower portion 86. Eachportion 85 and 86 defines teeth 87 for meshing with each other.

The upper portion 85 of the spline connection 84 is fixed onto spindle54; the lower portion 86 of the spline connection 84 is axiallyslideable on shaft 83. Rotation of portion 86 with shaft 83 isaccomplished by sliding key 91 mounted through the spline connection 84and inserted into a longitudinal slot or keyway 83' for example machinedinto the shaft 83 itself. The sliding key 91 permits axial movementbetween the motor shaft 83 and the lower portion 86 of spline connection54 sufficient to accommodate axial movement of lower portion 86 frombetween the compressed and decompressed positions of biasing member orspring 94. A biasing member, shown as a spring 94 bears against aradially extending flange at the upper end of lower portion 86 of thespline connection 84. (Suitable bearing means, such as a rollerbearing--not shown, are provided between the upper end of spring 94 andthe flange of lower portion 86 to facilitate rotation of the lowerportion 86 with respect to spring 94.) Thus spring 94 urges upward andsupports lower portion 86 which in turn normally supports the weight ofspindle 54, drive piston 52 and the control rod 16' coupled to thelatter.

Before removing the motor support structure 38 from the control roddrive 16 during disassembly, the drive piston 52 is uncoupled from thecontrol rod 16'. This is accomplished by withdrawing the control rod 16'from the core 17 by turning the spindle 54 until the control rod 16' isback-seated against a sealing surface 93 at the bottom of the pressurevessel. This back-seated condition is shown at the top of FIG. 2.

Continuing to turn the spindle 54 after the control rod 16' isback-seated, causes an upward movement of the spindle 54 as theupwardly-biasing member or spring 94 decompresses against the lowerportion 86 of the spline 84. The upper portion of the spindle 54 thenmeets the uncoupling rod 77 and moves it vertically and displaces waterin the upper portion of chamber 79, ejecting it through passage 82. Thisintroduces other water into the lower portion of chamber 79 throughpassage 81. This does not lead to overcompression in region 82', becausethe ball nut 53 does not serve as a barrier to the passage of water. Theuncoupling rod 77 upwardly displaces the plug 75, permitting the fingersof the spud 71 to move inwardly, releasing the drive piston 52 andallowing it to translate downwards.

By removing nuts 43, withdrawal is permitted of the motor supportstructure 38 and the motor 24 from the underside of the control roddrive 16. As withdrawal occurs, seals 95 are effective to preventspillage of water until preferably spherically-shaped surface 97 of thespindle 54 and the preferably conical mating inner surface 98 of housingstructure 22 meet to prevent leakage therethrough. Surfaces 97 and 98may have other shapes to effect sealing operation. Other seals 99 in asuitable housing 100, which supports the biasing member or spring 94,effectively prevent leakage along the shaft 83 of motor 24. Surface 97defines protrusions 102 effective for cooperating with recesses 101 insurface 98. This prevents rotation between surfaces 97 and 98 once theyare engaged. By inhibiting spindle rotation in this manner, accidentalwithdrawal of the control rod 16' from core 17 is prevented.

After the motor support structure 38 has been removed, the housingstructure 22 can be removed as well. This involves undoing studs 41 byworking the enlarged portion 42 as for example with a wrench andsuitably withdrawing the guide tube 59 and its contents from the outertube 21, possibly with the assistance of a mechanical hoist (not shown)and the transport cart 28 as is for example shown in FIG. 1.

The foregoing description is susceptible of reasonable modificationsthat may occur to those skilled in the art. However, this invention isnot meant to be limited to the embodiment just shown and described. Theclaims set forth the inventive concepts and are intended to cover allmodifications coming within the spirit and scope of the inventiondescribed herein.

What is claimed is:
 1. In a vertically oriented control rod drive devicepositioned beneath a pressure vessel for selectively inserting a controlrod into a nuclear reactor core in said pressure vessel including atubular drive housing secured to the bottom of said pressure vessel andextending downward therefrom; a lower housing secured to and closing thelower end of said drive housing; a drive piston having an upper headportion and a lower, elongated, annular skirt portion reciprocablydisposed in said drive housing including sealing means for providing afluid seal between said drive piston and said drive housing; an upwardlyextending coupling member secured to the top end of said drive pistonand including coupling means for releasably coupling said drive pistonto said control rod; a coupling release rod coaxially positioned in saidconnecting member and penetrating into said drive piston and beingreciprocable between an upper position and a lower position; latchingmeans secured to said coupling release rod for latching said couplingmeans to said control rod when said release rod is in its lower positionand for releasing said coupling means from said control rod when saidrelease rod is in its upper position; means for admitting a drive fluidat a pressure higher than the pressure in said pressure vessel to theunder side of said drive piston for rapid fluid drive insertion of saidcontrol rod into said core; a stabilizing arrangement for preventingmovement of said coupling release rod toward its upper position andconsequent release of said coupling means during fluid drive of saiddrive piston comprising: a cylindrical chamber in said drive pistonsurrounding said release rod; an annular chamber piston in said chambersecured to said release rod for reciprocating therewith; a first fluidpassage from the under side of said drive piston to the upper end ofsaid chamber; and a second fluid passage from the upper end of saiddrive piston to the lower end of said chamber whereby during fluid driveof said drive piston the net force on said chamber piston is downward tomaintain said release rod in its lower position whereby said control rodremains coupled to said drive piston.